As energy source prices are increasing due to depletion of fossil fuels and interest in environmental pollution is escalating, demand for environmentally-friendly alternative energy sources is bound to play an increasing role in future life. Thus, research into various power generation techniques such as nuclear energy, solar energy, wind energy, tidal power, and the like, continues to be underway, and power storage devices for more efficient use of the generated energy are also drawing much attention. As these power storage devices, secondary batteries are mainly used. Among these, in particular, lithium secondary batteries are mainly used in portable devices, demand therefor increases due to lightweight, high voltage, and high capacity, and use of lithium secondary batteries continues to expand to applications such as electric vehicles, hybrid electric vehicles, and auxiliary power supplies through smart-grid technology.
However, to use lithium secondary batteries as high-capacity power sources, many challenges that need to be addressed remain, and the most important challenge is improvement in energy density and safety. In addition, uniformity of wetting due to large-scale area and reduction in manufacturing time are also one of the most important challenges to be addressed. Therefore, many researchers have put spurs to research into materials that have enhanced energy density and are manufactured at low cost and also put much effort into research on materials for enhancing safety.
As materials for enhancing energy density, Ni-based materials, Mn-based materials, and the like having higher capacity than that of conventionally used LiCoO2 have been studied, and research into materials for forming an anode through Li alloy reaction instead of conventional intercalation reaction using Si, Sn, or the like, not using existing graphite-based materials is underway.
To enhance safety, research into a stable olivine-based cathode active material such as LiFePO4, an anode active material such as Li4Ti5O12, or the like is underway. However, such materials for enhancing safety fundamentally have low energy density and do not fundamentally address problems in terms of safety, caused due to structures of lithium secondary batteries.
Safety of secondary batteries may largely be classified into internal safety and external safety and further classified into electrical safety, impact safety, thermal safety, and the like. In these various safety problems, temperature increases when problems occur and, in this case, contraction of generally used stretching separators inevitably occurs.
Therefore, many researchers have proposed all solid type batteries to address safety problems, but such batteries cannot replace commercially available batteries due to several problems thereof.
First, currently used electrode active materials are in a solid state and, when a solid electrolyte or a polymer electrolyte is used, a contact surface between the solid electrolyte or the polymer electrolyte and the electrode active material for migration of lithium is very small. As a result, although a solid electrolyte or a polymer electrolyte itself has a conductivity of 10−5 S/cm, which corresponds to conductivity of a liquid electrolyte, ionic conductivity thereof is very low.
Second, for such reason, ionic conductivity occurring at an interface between solids or an interface between a solid and a polymer is inevitably further reduced.
Third, adhesive strength is important in manufacture of a battery and, even if a solid electrolyte with high conductivity is used, it is necessary to use a polymer binder, which causes further reduction in ionic conductivity.
Fourth, to manufacture a battery, only a separation layer does not require ionic conductivity. To enhance ionic conductivity of an electrode, cathode and anode active materials also require materials for enhancing ionic conductivity and, when a solid electrolyte or a polymer electrolyte is used as an electrode component, capacity is reduced.
Fifth, when a polymer electrolyte including an organic electrolyte is used, mechanical/physical properties and ionic conductivity have trade-off relationship and thus, when the amount of the organic electrolyte is increased to enhance ionic conductivity, mechanical/physical properties of a polymer electrolyte layer are significantly deteriorated.
Therefore, there is a high need to develop a battery that prevents short circuit due to contraction of a separator and has excellent electrical performance.